Link "DNNNN" and "TNNNN" in Phabricator remarkup.

Summary: Autolink Differential and Maniphest objects.

Test Plan: Typed "D12345" and "T12345" into the Differential comment preview,
got links. Typed "http://www.elsewhere.com/D12345" and got a single link to
that URI, not a mess where the D12345 part linked incorrectly.

Reviewers: aran

CC:

Differential Revision: 35